Medical ultrasonic diagnostic apparatuses allow easy observation of the inside of the patient body in real time while imposing a light burden on the body, and hence are used in various fields of clinical scene. An ultrasonic diagnostic apparatus displays an image with luminances corresponding to the intensities of reflected echoes. The intensity of a reflected echo is decided by a beam angle onto the boundary surface between tissues such as a joint and a blood vessel and the acoustic impedances unique to the tissues. For this reason, if an ultrasonic beam (transmitted ultrasonic wave) does not vertically insonified a body tissue, since the intensity of the reflected echo is weakened, the tissue may not be accurately shown. In addition, visibility is impaired by speckle noise or random noise in the observation, which is generated by interference with the reflected echo or scattered wave.
In order to solve the above problem, for example, there is available a technique of transmitting/receiving ultrasonic waves in different beam directions, evaluating the anisotropy of the reflection of ultrasonic waves from obtained signals or the image data generated from the signals, and compounding images based on the anisotropy.
In addition, generated images differ in the continuity of body tissues and speckle noise patterns depending on the frequency bands of received ultrasonic beams. For example, an image obtained with a low-frequency beam is high in the continuity of a body tissue, while speckle noise is frequently observed to impair the visibility of the image. Speckle noise is not noticeable on an image obtained with a high-frequency beam. However, since the continuity of the body tissue is low, the body tissue as a diagnosis target is displayed as a non-contiguous image. This disturbs diagnosis.
There is available a technique of simultaneously receiving reflected beams having different frequencies and performing weighted averaging of the two reflected beams to solve the above problem.
However, the technique of evaluating the anisotropy of the reflection of ultrasonic waves from obtained signals or the image data generated from the signals and compounding images based on the anisotropy is designed to calculate the anisotropy using a single pixel. For this reason, when evaluating anisotropy on an ultrasonic image with a lot of noise, the influence of noise will lead to erroneous evaluation. In this case, noise is enhanced instead of the tissue, resulting in a failure to sufficiently depict the tissue.
In addition, the technique of simultaneously receiving reflected beams of different frequencies and performing weighted averaging of the two reflected beams is designed to combine high and low frequency components by simple weighted averaging. The resultant image is therefore an intermediate compound image. That is, this technique does not take full advantages of the respective frequency components.